Big bang breakthrough: Who is the father of inflation?

The first clear glimpse of ripples in space-time, known as gravitational waves, is being widely hailed as validation for inflation, the notion that the baby universe ballooned in size mind-bendingly fast just after the big bang. Reported last week, the discovery may earn some scientists a Nobel prize if confirmed by further experiments. But who are the founders of inflation?

Like the Higgs boson, which was hypothesised in various forms by several groups around the same time, it turns out that inflation has many fathers. That's partly because it draws on many disparate ideas in physics and cosmology.

Alan Guth, now at the Massachusetts Institute of Technology, is widely quoted as the father of inflation. He presented his ideas in 1980, making him the first in the Western world to present the notion of the ballooning universe.

Since then, physicists have suggested a plethora of different ways that the universe may have done this. Guth's particular version did not work well in a mathematical sense as it predicted defects in space-time that are not visible in the universe – though a few years later Andrei Linde of Stanford University in California removed these defects. The result is what is now called chaotic inflation, which is driven by quantum processes that constantly trigger new bursts, giving rise to multiple universes. This model also fits very well with the results presented last week, by the BICEP2 gravitational-wave experiment based in Antarctica: Linde was very pleased by the result. However, chaotic inflation does not fit well with some other cosmological observations.

Russian scoop

But Guth was not the only one pondering a cosmic growth spurt at that time. In December 1979, Russian physicist Alexei Starobinsky, now at the Landau Institute for Theoretical Physics in Moscow, independently published his theory of inflation. He predicted that a spectrum of primordial gravitational waves would be produced from any inflationary universe, regardless of the model.

Starobinsky's paper appeared in a prominent Russian journal, JETP Letters, which was also translated into English. It came three years before similar work on gravitational waves in the West. But it failed to make much of an impact among scientists, at least until very recently. Starobinsky was awarded the Gruber cosmology prize in 2013 – Guth and Linde won it in 2004.

Viatcheslav F. Mukhanov, now at the Ludwig Maximilian University of Munich in Germany, followed up on Starobinsky's work in 1981, along with another Russian scientist. Mukhanov thinks Starobinsky was less well-known than Guth because he came at inflation from a different angle.

Guth used particle physics, a well-known playground for theorists, to express his ideas, inspiring Western scientists to look for inflationary scenarios that might arise from the particle soup that existed shortly after the big bang. By contrast, Starobinsky was trying to understand how the big bang itself may have happened. To do this, he was investigating how Einstein's general theory of relativity might apply in compact, high-energy situations.

We now know that both approaches are essentially the same. An as-yet unknown quantum theory of gravity should have been working in the universe at the energy scales present during cosmic inflation. Such a theory would unite the two ideas.

Impossible test

But at the time, those working on relativity wanted predictions that could be tested experimentally, and any test for inflation seemed almost impossible, says Mukhanov. Guth's ideas, meanwhile, sparked widespread interest as a mathematical exercise.

Until gravitational waves came on the scene in the form of the BICEP2 result, Starobinsky's model seemed to fit our observations of the early universe better. "Without the BICEP data, Starobinsky's would remain the best model to fit the available data today," says cosmologist David Wands at the University of Portsmouth, UK. In the wake of the BICEP2 result, it is unclear which theory of inflation will reign (see "Space-time ripples hint at physics beyond the big bang").

"If the BICEP2 data are confirmed, apostles of inflation, like Andrei Linde or me, may celebrate the dramatic confirmation of the general idea of inflation, but not of any concrete simple model of it," Starobinsky told New Scientist. "On the other hand, a new era of gravitational-wave astronomy of the early universe begins: now one can investigate the fine structure of inflation."

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